1. Field of the Invention
The present invention relates to a method of manufacturing an optical element having annular zones centered about an optical axis and a method of manufacturing a mold for forming an optical element having annular zones to mold the same.
2. Description of the Related Art
In general, an optical element such as a Fresnel lens and a blaze type diffraction optical element having a spherical face centered about an optical axis and having a plurality of minute, protruding annular zones formed around a periphery of the spherical face thereof, is manufactured using a mold having formed therein a concave portion corresponding to the spherical portion of the optical element and annular grooves corresponding to the minute annular zones of the optical element. As a general method of manufacturing such a mold with annular grooves, there has been known a method of cutting the minute annular grooves by means of a tip. In a case of an optical element such as a phase shift element having a circular face centered about an optical axis in plan view and having a plurality of annular zonal faces formed in a step-like manner around a periphery of the circular face thereof, a turning process is directly provided for the optical element by means of a tip and the like. In a method of cutting by means of a tip such an optical element having annular zonal faces thereon formed in a step-like manner or a mold having above-mentioned annular grooves, since a front end shape of the tip is transferred onto each angle portion of the steps of the optical element or each angle portion of the annular grooves of the mold, it is desirable to make the front end shape of the tip acutely pointed. However, if the front end shape of the tip is made acutely pointed, this may cause deteriorating the surface roughness of the processed face.
Conventionally, the front end of a tip 21, which is a cutting tool, has a round form with a minute radius (with a minute R) as shown in FIG. 6 to form minute annular grooves in a mold, as disclosed in JP-A-2003-62707 (Paragraph 0059, FIGS. 3 and 6). Specifically, with a rake face 21a of the tip 21 set perpendicular to a rotation direction of a solid cylindrical workpiece W, the tip 21 is moved at an appropriate timing in the axial direction of the workpiece W while being moved continuously along a radial direction of the cylindrical workpiece W, whereby allowing the front end of the tip 21 to trace along wave-like processed faces PF which has been roughly processed, so as to form minute annular grooves 7 as shown in FIG. 7. According to the above-mentioned method, the surface roughness is more improved in comparison to a case where the front end of the tip 21 is sharply pointed, and a bottom portion of each annular groove 7 can be formed to be of a minute, rounded shape. The above method can be considered as applicable to an optical element of which annular zonal faces are formed in a step-like manner, as well.
However, although the above-mentioned method can improve the quality of the surface roughness somewhat higher than the case where the front end of the tip 21 is sharply pointed, the surface roughness still does not reach a desirable level, compared to cutting quality in use of a conventional-type tip. There is another method of cutting using a tip at an extremely low feeding speed thereof, so that a surface roughness can be as preferable as possible. However, this method increases the processing time, and may cause lowering of the temperature stability in the processing environment, resulting in a high possibility of deterioration in the resulting shape. Furthermore, since the tip has a narrower front end, abrasion of the tip becomes greater in comparison to a conventional-type tip.
Thus, it is an object of the present invention to provide a method of manufacturing an optical element having annular zones and a method of manufacturing a mold therefor, whereby the surface roughness can be improved without taking up more time.